Induction furnace with thermocouple assembly

ABSTRACT

An induction furnace for melting metal has a built-in thermocouple assembly for continuously measuring the temperature of the molten metal. A power controller then adjusts the power to the furnace to maintain a predetermined temperature. The thermocouple assembly includes a refractory substrate which forms a portion of the furnace lining partially coated with thin strips of two thermoelectric materials so as to form a thermocouple junction. The refractory substrate and the strips of thermoelectric materials are covered with a protective layer of the same refractory material as the substrate. Electrical leads are attached to the assembly and are in contact with the strips of thermoelectric material. An alternative embodiment utilizes a similarly constructed thermocouple which is immersed in the molten material for intermittent measurements.

United States Patent 1191 1111 3,715,441

Collins Feb. 6, 1973 UCT ON FURNAC WITH Primary Examiner-Roy N. Envall,J r.

THERMOCOUPLE ASSEMBLY Attorney-Richards, Harris & Hubbard [76] Inventor:Henry F. Collins, 917 Alamo, Gar- [57] ABSTRACT land, Tex. 75040 [22]Filed: July 26 1971 An induction furnace for melting metal has abuilt-in thermocouple assembly for continuously measuring [21] App].No.: 166,009 the temperature of the molten metal. A power controllerthen adjusts the power to the furnace to main tain a predeterminedtemperature. The thermocouple 5 2 1 U 8 Cl 13/35 fg assembly mcludes arefractory substrate which forms a portion of the furnace liningpartially coated with thin {g 2 5 2 9 strips of two thermoelectricmaterials so as to form a thermocouple junction. The refractorysubstrate and 29/592 the strips of thermoelectric materials are coveredwith a protective layer of the same refractory material as [56]References Cited the substrate. Electrical leads are attached to the as-UNITED STATES PATENTS sembly and are in contact with the strips ofthermoelectric material. An alternatwe embodlment ut1l- 2,519,941 5 Tama..l3/26 UX izes a similarly constructed thermocouple which is im-2,655,550 10/1953 Zvanut mersed in the molten material for intermittentmea- 3,006,978 10/1961 McGrath et al. suremems 81 12/1963 Shearman..l36/233 X 5 Claims, 13 Drawings POWER CONTROLLER Z6 PATEHTEDFEB 6l9753,715,441 SHEET 10F 4 POWER CONTROLLER INDUCTION CORE FIG!

INVENTORI HENRY E COLLINS ATTORNEYS PATENTED FEB 6 I975 SHEET 30F 4 FIG.8

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I I 9 l 4 1 9 x z 1 r I I l I I I 7 /7 HENRY F. COLLINS ATTORNEYSINDUCTION FURNACE WITH THERMOCOUPLE ASSEMBLY BACKGROUND OF THE INVENTIONsembly having very thin strips of thermoelectric material deposited on arefractory substrate and covered by a protective layer of refractorymaterial.

It is important in metal pouring to control the temperature of themolten metal as closely as possible to an optimum temperature. Failureto achieve temperature control can result in inferior molded productsbecause of variations in the alloying constituents of the metal beingcast. Many attempts have been made to perfect a system for measuring thetemperature of molten metal within a furnace with a notable lack ofsuccess. The solution to this problem is made very difficult because ofthe extremely hostile environment within the furnace. Not only are thetemperatures very high, but the surface of the molten material isusually covered with oxides of the metal and oxides of impurities, whichare very corrosive. In areas of the furnace where stirring occurs, themetal itself is highly abrasive. Only very special refractory materialsare suitable for direct contact with the molten material. As a result,various devices have been proposed for measuring these temperatureswithout directly contacting the molten metal, without notable acceptanceby the industry. The thermocouple has also been immersed directly intothe molten material by various means.

A typical thermocouple device for the thermoelectric measurement oftemperature utilizes two wires of different materials which are weldedtogether at one point. This point, called the hot junction, is placed asnear as possible to that which is to be monitored. The other two wireends, or cold junction, are connected to a measuring instrument. In sucha closed circuit a current will flow continuously when the two junctionsare maintained at different temperatures.

Prior art thermocouple devices which have been installed in inductionfurnaces have been unsuccessful in continuously and accurately measuringthe temperature of molten metals. A device which is installed in afurnace wall will not give continuously accurate readings unless it islocated so close to the metals that the furnace lining is weakened andthe protective sheath quickly destroyed. Likewise, commerciallyavailable immersion devices give fairly accurate intermittentmeasurements but are soon destroyed by corrosion and abrasion and haveto be discarded. Such rapid deterioration of thermocouple assembliesindicates the importance of devising long-lived and inexpensivethermocouple assemblies.

In thermoelectric devices of the prior art used for measuring thetemperatures of molten metals the assembly had to be protected from theheat source. Commonly, the two wires were inserted through a refractoryor dielectric insulator, welded or joined together, and inserted into aprotection tube ofa suitable material. A terminal assembly was thenattached to the loose wire ends. In one commercially available device, athermocouple is inserted in a U-shaped quartz tube which is immersed inthe molten metal. Although the quartz is dissolved in the molten iron,the quartz tube delays contact of the metal with the thermocouplesufficiently long to give a temperature reading. These devices aretherefore usually one-shot expendable devices and are therefore notsuited for use as the sensor in a continuous control system.

SUMMARY OF THE INVENTION In the present invention a vessel forcontaining molten metal includes a thermocouple device utilizing thinstrips of thermoelectric materials deposited on a refractory substrateto from a thermocouple junction and covered by a layer of protectiverefractory material.

More particularly, in the preferred embodiment of the present invention, there is provided a vessel for containing molten materials inwhich a refractory member forms a part of the refractory lining of thevessel and has a surface exposed to the molten materials for measurementof the temperature of the melt. Thin strips of two thermoelectricmaterials coat a portion of the refractory member and contact each otherto form a thermocouple junction. A protective layer of refractorymaterial, preferably the same material as that used for the refractorymember, covers the thermocouple materials and the refractory member.Electrical leads are attached to the strips of thermo-electric materialsand extend outside the protective covering to a measurement device.

The thermoelectric materials may be deposited upon the refractory memberin very thin strips on the order of l X l0 inches by a sputtering orevaporative coating process. Likewise, the protective coating, which maybe as thin as 4 X 10' inches, may also be deposited by sputtering or bya flame spray. The layers so deposited bond tightly with the refractorymember, particularly if the refractory layer is the same material as thesubstrate.

The thermoelectric assembly is installed in the wall of an inductionfurnace or a molten metal ladle for the purpose of measuring thetemperature of the molten material therein. The measured temperature maybe used to automatically control the temperature of the metal. In analternative embodiment, the thermocouple assembly may be used as aseparate immersion device which is dipped into the molten materialsafter the materials are melted.

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding ofthe present invention and for further objects and advantages thereofreference may now be had to the following description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a cross-sectional side view of a channel-type inductionfurnace showing the thermocouple assembly installed;

FIG. 2 is an enlarged cross-sectional partial side view of the furnacein FIG. 1 showing the relationship of the thermocouple assembly to themolten metal during pouring;

FIGS. 3, 4, 5 and 6 are the side, top, front and back cross-sectionalviews, respectively, of one embodiment of the thermocouple assembly;

FIG. 7 is a cross-sectional side view sketching a drum-type inductionfurnace with thermocouple assembly installed;

FIG. 8 is a cross-sectional side view of a coreless-type inductionfurnace showing the installation of the thermocouple assembly;

FIG. 9 is an enlarged cross-sectional partial side view of the furnacein FIG. 8 showing an alternate installation of an easily removablethermocouple assembly P g;

FIG. 10 is a perspective view of the thermocouple assembly plug;

FIG. 11 is a cross-sectional side view sketch of a channel-typeinduction furnace using an alternative embodiment of the invention inthe form of a thermocouple immersion device;

FIGS. 12 and 13 are cross-sectional side and front views, respectively,of the thermocouple immersion device embodiment.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIG. 1, apparatusin accordance with the present invention is indicated generally by thereference numeral 8. The apparatus 8 includes a channel-type inductionfurnace 10. Thermocouple device 12 is shown as one of the bricks in therefractory wall lining 14 of the induction furnace 10. Channel 16 leadsto a heating core 17 where heat is supplied by induction means formelting materials in furnace 10. The thermocouple device 12 is showninstalled in wall 14 below the minimum level 18 of melt 22 so as toavoid the corrosive conditions that exist at the interface between thesurface of the molten materials 22 and the surrounding atmosphere. Athermocouple junction is positioned within thermocouple device 12 nearthe surface 20 which is exposed to molten materials 22.

Electrical leads 24 are attached to thermocouple device 12 and extendthrough the outer refractory wall 26 of furnace 10 to a thermohead 28.The leads 24 are then attached to a suitable control circuit 29 whichmay provide a reading of the temperature and also automatically controlthe power in the core 17.

In FIG. 2, a part of induction furnace 10 is shown in a tilted position,indicating the position of thermocouple device 12 in relation to melt 22during pouring. As shown, device 12 is installed in the wall 14 suchthat the exposed surface 20 of thermocouple device 12 remains covered bythe melt 22 during the pouring operation.

Having shown the thermocouple device 12 in one type of installation,description will now be made in FIGS. 3-6 of the construction of thethermocouple assembly. As shown in FIG. 3, a substrate of refractorymaterial 30 has connecting leads 32 which are fixed in grooves in thesubstrate and have terminal strips 34, best seen in FIG. 4.

Very thin strips of thermoelectric material 36 partially coat one sideand end of the substrate 30. A second thermoelectric material 38partially coats the opposite side of substrate 30 and that portion ofthe first thermoelectric material 36 on the end of substrate 30, thusforming a thermocouple junction 40. The thermoelectric materials 36 and38 and the exposed portion of substrate 30 are covered with a coating ofrefractory material 42 which is the same or similar to the material ofsubstrate 30.

For clarity, the strips of first and second thermoelectric materials 36and 38, as shown in the drawings, are considerably exaggerated inthickness. In practice, the thermoelectric materials are preferablydeposited in strips as thin as I X 10' inches on substrate 30 by asputtering or evaporative coating process. Refractory coating 42, whichis preferably on the order of 4 X [0' inches thick, may also bedeposited by a conventional sputtering or flame spray process, hereafterdescribed in greater detail.

FIG. 4 is a top view of the thermocouple assembly showing five strips ofsecond thermoelectric material 38 deposited on substrate 30. The strips38 are commonly connected by a terminal strip 34 to an electrical lead32. The strips of the first thermoelectric material 36 are likewisecommonly connected by a terminal strip 34 to an electrical lead 32 asbest shown in FIG. 6.

A number of pairs of strips 36 and 38 may be deposited on substrate 30.Preferably, a considerable portion of substrate 30 would remain exposedfor direct bonding to the refractory coating 42, to minimize thepossibility of a heat fracture in the thermocouple assembly 12.

In FIG. 5, the thermocouple junction 40, formed by the overlapping ofstrips 36 and 38, is shown deposited in a lattice-type configuration onsubstrate 30. Such a configuration allows the protective layer ofrefractory 42 to bond directly to substrate 30 over a considerableportion of the surface area. This design promotes bonding and preventsseparation of the thermoelectric strips 36 and 38 from the refractorylayer 42 or the substrate 30 due to differences in coefficients ofthermoexpansion. When platinum and platinum (9O percent)-rhodium (10percent) are used as the thermoelectric materials, the rhodium tends todiffuse into the pure platinum. The lattice-type design extends theusable life of the assembly by creating relatively long diffusion pathsfor the rhodium. FIG. 6 shows the common connections of thethermoelectric strips 36 and 38 by terminal strips 34.

In FIG. 7, the thermoelectric assembly 12 is shown installed in adrum-type induction furnace 50. As in the channel induction furnace 10,the thermoelectric device 12 is installed in the refractory lining 52 offurnace 50. Lining 52 is not shown in detail in FIG. 7 but may becomposed of other bricks of the same size as thermoelectric assembly 12or some other similar refractory material. As in the channel furnace 10,it is advantageous that the thermoelectric device 12 be installed indrum furnace 50 in a position to remain below the minimum level (notshown) of melt 54 as much as possible. Electrical leads 56 are connectedto device 12 and extend through outer refractory layer 58 to thermohead59.

FIG. 8 shows a smaller coreless-type induction furnace 60. In furnace60, the inner lining 62 is composed of a tamped or rammed refractorysuch as aluminum oxide (M 0 and forms the inner walls and bottom of thefurnace 60. An inner refractory brick layer 64 is laid below the rammedlining 62 forming the inner bottom of furnace 60. An outer refractorylayer 70, also composed of brick, lies below inner layer 64 and directlyabove the furnace bottom plate 74.

The coreless-type induction furnace 60 is often emptied of all moltenmaterials 66. The thermocouple assembly 12 is thus located in the baseof furnace 60 so that the assembly 12 is covered by the melt 66 as muchas possible, to reduce corrosive activity. The thermocouple device 12 isembedded in the refractory lining 64 and projects through the innerlining 62 to expose the thermocouple junction face to melt 66.

82 is secured against furnace bottom plate 74 by bolts 84 thus securingplug 80 in position in the bottom wall of furnace 60. Plug 80 may beeasily removed and a new plug installed whenever corrosive activitydeteriorates the thermocouple assembly 12 to the point where thethermocouple junction is no longer operative. Electrical leads 88 extendthrough outer lining 70 to a thermohead 89.

FIG. 10 shows a perspective view of the replaceable thermocouple plug80. As shown, the thermocouple assembly 12 is mounted on an oversizedbrick 86 which in turn is mounted on face plate 82. Electrical leads 88follow the edge of oversized brick 86 and couple to the thermohead 89shown in FIG. 9.

Another alternate embodiment of the invention is shown in FIG. 11, inwhich an immersion device 90 is lowered into a melt 92 of a channel-typeinduction furnace 10 for measurement of the melt temperature. Theimmersion device 90 is lowered by a rod 94 or some other means throughan opening 93 in the top of induction furnace 10 or through the open topof furnace 10. Electrical leads extend inside rod 94 to connect toimmersion device 90.

One form of construction of the immersion device 90 is shown in FIGS. 12and 13. A strip of a first thermoelectric material 94 is deposited on asubstrate of refractory material 98. The strip of material 94 par tiallycovers a part of one side and one end of substrate 98. A strip of asecond thermoelectric material 96 partially coats the opposite side ofsubstrate 98 and overlaps the end portion of strip 94 to form athermocouple junction 99. Electrical leads 97 are attached to strips 94and 96 and are connected to a suitable measuring device (not shown).

Thus, the construction of immersion device 90 is similar to that shownfor the thermocouple assembly 12 in FIGS. 3-6 except that only one pairof thermoelectric strips is used for immersion device 90. A protectivelayer of refractory material 95 also covers the thermoelectric strips 94and 96 and the exposed substrate 98. Although only one set of strips isshown, several strips may be used in constructing the immersion device90 as in the thermocouple assembly 12.

Another alternative embodiment of the replaceable thermocouple plug ofFIGS. 9 and 10 utilizes a multiple junction thermocoupled assembly inwhich thermocouple junctions are sequentially sputtered, one uponanother, and separated by a protective coating of refractory material.As the surface of the protective refractory material is exposed to boththe melt and the atmosphere, it eventually erodes, and the firstthermocouple junction is destroyed. The next junction located behindanother protective layer of refractory material is then manually orautomatically switched on for monitoring the temperature. This procedureis continued until all junctions are destroyed, at which time themultiple junction plug is replaced.

In the sputtering process, the coatings are preferably applied in achamber which has been evacuated to a high level of vacuum, usually inthe range of 10 Torr. The chamber is then backfilled to a partialpressure of approximately 6 to 20 X 10' Torr with a gas, preferably aninert gas such as Argon. A refractory substrate member, with electricalleads and terminal strips attached, is placed in the vacuum chamber.

As shown in FIG. 3, the thermoelectric strips 36 are sputtered on a sideand an end of substrate 30. Strips 38 are then sputtered on the oppositeside of substrate 30. Strips 38 are also deposited over the portions ofstrips 36 coating the end of substrate 30, thus forming a thermocouplejunction. Finally, a protective coating 42 of refractory material,similar to or the same as the material of substrate 30, is sputteredover the thermoelectric strips 36 and 38 and the exposed portion ofsubstrate 30.

The sputtering process is performed using conventional sputteringapparatus and procedures, such as those described in an articleentitled, Sputtering: Electronics, Razor Blades, Then What?" ProductFinishing, March, 1970. The coatings are applied slowly, molecule bymolecule, resulting in a very high bonding strength. For all practicalpurposes, the sputtered coatings become part of the substrate material.

The thermoelectric materials to be used are often expensive metals suchas platinum or rhodium. The sputtering process greatly reduces the costof production by depositing very thin films of the expensive metals.Furthermore, if the thermocouple junction and the thermoelectric stripsare eventually destroyed the remaining cavities are too thin to providea passage for the flow of molten metal. Although sputtering is preferredfor depositing the thin strips of thermoelectric materials, anotherprocess which will result in the depositing of strips as thin or thinnerthan those deposited by sputtering may be used without departing fromthe broader aspects of the invention.

Sputtering is also preferred for coating the thermocouple assembly withan initial protective refractory layer, because of the excellent bondingresulting from the sputtering process. However, it is sometimesdesirable to build up a heavier protective layer than is practical usingsputtering. For example, a protective coating of 0.030 inches may berequired. Since this would require several days using sputteringapparatus presently available, the protective refractory layer may bethickened using a flame or plasma gun or other suitable device.

Although the thermocouple assembly has been shown installed in inductionfurnaces, the assembly may be used in other vessels or apparatus fortemperature measurement without departing from the scope of theinvention. For example, the thermocouple assembly may be installed in aladle for temperature measurement of the hot or molten materialstherein. The thermocouple device may also be employed in other types ofheating devices where temperature measurement is desired.

From the foregoing the advantages of the present invention are readilyapparent. Using the invention, it is possible to obtain accurate andcontinuous temperature measurements of molten materials. The thinness ofthe thermoelectric strips provides for an extremely rapid response tochanges in temperature. The protective layer of refractory material isalso sufficiently thin so as not to significantly affect response time.

Bonding the strips to a strong substrate by sputtering results in thetensile strength of the conducting strips being essentially that of thesubstrate. Employing a refractory material similar to the substrate forthe protective layer of the device gives strong bonding and resistanceto heat fracturing of the thermocouple assembly.

The thinness of the thermoelectric strips significantly reduces theamount of conductive material needed and thus reduces production costs.Furthermore, if the strips are destroyed, the molten materials cannotleak through the furnace walls.

Having described the invention in connection with certain specificembodiments thereof, it is to be understood that further modificationsmay now suggest themselves to those skilled in the art. It is intendedto cover such modifications as fall in the scope of the appended claims.

What is claimed is:

1. The combination comprising:

a system for heating molten metals including a vessel having arefractory lining for containing molten metal,

a substantially solid refractory substrate having one face forming aportion of the interior surface of the refractory lining,

a thin layer of a first thermo-electric material deposited on and bondedto an exterior surface of the substrate,

a thin layer of the second thermoelectric material deposited on andbonded to a substantial portion of the layer of the first thermoelectricmaterial to form a thermocouple,

electrical conductor means extending from each of the thin layers of thefirst and second thermoelectric materials along a surface of therefractory substrate to a point remote from the face forming a portionof the interior surface of the refractory lining, and

a relatively thin layer of refractorymaterial deposited on and coveringthe exposed surfaces of the first and second thermoelectric materialsand the electrical conductors and at least a portion of the substrate,the refractory material being bonded to at least a portion of thesubstrate as a result of the deposition process.

2. The combination of claim 1 wherein:

at least a portion of the refractory lining is formed by a plurality ofrefractory bricks, and wherein the refractory substrate is placedbetween a plurality of the bricks with one face of the substrate beingsubstantially planar and substantially coplanar with the interiorsurface of the vessel formed by the refractory bricks.

3. The combination of claim 1 wherein the thermocouple is formed on theface of the substrate that is substantially coplanar with the interiorsurface of the refractory lining.

4. The combination of claim 1 wherein the thin layer of refractorymaterial is substantially the same material as the refractory substrate.

5. The combination of claim 4 wherein the thermoelectric materials aredeposited in a series of thin strips forming a lattice network.

1. The combination comprising: a system for heating molten metalsincluding a vessel having a refractory lining for containing moltenmetal, a substantially solid refractory substrate having one faceforming a portion of the interior surface of the refractory lining, athin layer of a first thermo-electric material deposited on and bondedto an exterior surface of the substrate, a thin layer of the secondthermoelectric material deposited on and bonded to a substantial portionof the layer of the first thermoelectric material to form athermocouple, electrical conductor means extending from each of the thinlayers of the first and second thermoelectric materials along a surfaceof the refractory substrate to a point remote from the face forming aportion of the interior surface of the refractory lining, and arelatively thin layer of refractory material deposited on and coveringthe exposed surfaces of the first and second thermoelectric materialsand the electrical conductors and at least a portion of the substrate,the refractory material being bonded to at least a portion of thesubstrate as a result of the deposition process.
 2. The combination ofclaim 1 wherein: at least a portion of the refractory lining is formedby a plurality of refractory bricks, and wherein the refractorysubstrate is placed between a plurality of the bricks with one face ofthe substrate being substantially planar and substantially coplanar withthe interior surface of the vessel formed by the refractory bricks. 3.The combination of claim 1 wherein the thermocouple is formed on theface of the substrate that is substantially coplanar with the interiorsurface of the refractory lining.
 4. The combination of claim 1 whereinthe thin layer of refractory material is substantially the same materialas the refractory substrate.